Use Laptop Mode to Save Power Based on Battery/AC Status

Laptop mode is a Linux kernel feature that allows your laptop to save considerable power, by allowing the hard drive to spin down for longer periods of time. This package contains the userland scripts that are needed to enable laptop mode.

It includes support for automatically enabling laptop mode when the computer is working on batteries. It also supports various other power management features, such as starting and stopping daemons depending on power mode, automatically hibernating if battery levels are too low, and adjusting terminal blanking and X11 screen blanking

laptop-mode-tools uses the Linux kernel's Laptop Mode feature and thus is also used on Desktops and Servers to conserve power

To install laptop-mode-tools, in a terminal type:
sudo apt-get install laptop-mode-tools

To read the laptop-mode-tools user manual, in a terminal type:

man laptop-mode.conf

To edit laptop-mode-tools settings, in a terminal type:

sudo mousepad /etc/laptop-mode/laptop-mode.conf

- if you aren't using mouspad as your text editor replace mousepad with the correct program